You are what you practice.
If you swing faster than full speed, that's how you'll swing when the pressure is on.
If you practice slower than full speed, that's how you're gonna swing when the pressure is on.
Now tell me, how should you practice?

The point of a shadow swing (for me anyway) is to help the player develop the feeling of firing the muscles in the correct sequence (in other words to myelinate [or insulate] nerve fibres to speed up the impulse from brain to muscle). As the level of myelin builds up, the impulses can travel faster and so the ability to fire becomes more efficient and therefore can happen faster whilst maintaining control.